Wide band gap semiconductor materials are used as photocatalysts for degrading dilute pollutants in air and water as well as for converting CO2 and H2O gases to valuable products such as hydrocarbons and H2 through oxidation and reductions (redox) reactions. A particular interest is the photocatalytic reduction of CO2 for the production of hydrocarbons and other valuable products using inexpensive and abundant semiconductors such as TiO2 and ZnO. Such processes provide a potential means to reduce atmospheric CO2, as well as providing an attractive alternative to purely chemical means of converting CO2 to hydrocarbons. However, a fundamental difficulty in the widespread use of many semiconductors such as TiO2 and ZnO is the requirement for ultraviolet light to drive photoexcitation. Because ultraviolet light constitutes a relatively small fraction of the solar spectrum, use of these materials for photocatalytic reduction of CO2 has generally required illumination by an artificial UV light source in order to generate sufficient redox capacity. This is a parasitic load to the system as a whole, and negatively impacts the efficiency of the process. As a result, shifting the optical response of these inexpensive and abundant semiconductor materials to provide for photoexcitation in the visible spectral range while preserving the ability to facilitate specific redox reactions is the subject of significant effort. In particular, providing a methodology by which wide band gap semiconductor materials could be effectively utilized for the photocatalytic reduction of CO2 as a response to visible light illumination would be of enormous benefit.
It is known that sensitization of semiconductors which respond primarily to UV light can provide a system whereby electrons excited by visible light in the sensitizer are injected into the semiconductor, and these systems have been extensively studied in applications such as solar cells and photography technology. In these systems, the band alignment of the sensitizer and the semiconductor is such that the energy level of the photoexcited electron within the sensitizer lies above the conduction band minimum of the semiconductor material. In operation, injection of electrons into the conduction band of the semiconductor material occurs upon visible light photoexcitation of the sensitizer. The bulk of the wide band gap semiconductor is used primarily for charge transport, and the arrangement further provides for charge separation in order to limit loss through recombination. Through this combination, semiconductors which respond primarily to UV light may be electronically coupled with a sensitizer, so that the response spectrum of the sensitizer-semiconductor system is extended into the visible light spectrum, and the properties of the semiconductor as an effective charge transporter can be utilized without artificial UV light sources.
Uses of these sensitizer-semiconductor configurations as redox systems driven by visible light have primarily been investigated in the degradation of dilute pollutants in air and water. Typically the underlying mechanism is described as the sensitizer injecting electrons to the semiconductor material as a result of exposure to visible light. These photoinduced electrons can transfer to molecular oxygen (O2), leading to the formation of active specials, such as superoxide/hydroxide radicals and singlet oxygen in the system. The repeated attack of these active species on the organic pollutant molecules results in their ultimate decomposition to carbon dioxide, water, and simple mineral acids. See e.g., Zhang et al, “Role of oxygen active species in the photocatalytic degradation of phenol using polymer sensitized TiO2 under visible light irradiation”, Journal of Hazardous Materials 163 (2009). These processes have been extensively investigated for the treatment of wastewaters and other pollutant-laden materials, however they rely on interaction between the injected electron and O2 for the purpose of converting pollutants to less harmful CO2, and do not address themselves toward the photocatalytic reduction of CO2 under the environmental conditions typically reported for utilization. It would be advantageous to provide a sensitized semiconductor for the photocatalytic reduction of CO2 under visible light illumination, in order to provide a potential means to reduce atmospheric CO2 as well as provide a mechanism for converting CO2 to hydrocarbons through more efficient use of the solar spectrum.
The use of dye sensitized platinized TiO2 has also been reported for the photocatalytic reduction of CO2 under illumination from a daylight lamp. See Ozcan et al., “Dye Sensitized CO2 reduction over pure and platinized TiO2”, Topics in Catalysis Vol. 44, No. 4 (2007). Methane production is reported under illumination by the daylight lamp, however daylight lamps provide illumination in both the UV and visible spectrum, and it is unclear whether the CO2 reduction for the production of methane reported results from UV or visible light irradiation. Additionally, dyes are known to degrade under UV light, and sensitizer-semiconductor systems such as solar cells which incorporate dyes for use under a typical solar spectrum typically include UV barriers to prolong service life. It would be advantageous to provide a methodology whereby a sensitized wide band gap semiconductor could rely solely on visible light for the photocatalytic reduction of CO2 without possible reliance on included UV wavelengths and direct photoactivation of the semiconductor material, so that the sensitized semiconductor could definitively utilize the greater fraction of the solar spectrum represented by the visible light. It would be further advantageous if the sensitized semiconductor could tolerate the presence of UV light in the utilized spectrum without reliance on UV barriers for prolonged service life. Additionally, dye molecules cannot undergo impact ionization and produce quantum yields greater than one, as has been reported for quantum dot sensitized solar cells. See Fu et al, “Impact Ionization and Auger Recombination Rates in Semiconductor Quantum Dots”, J. Phys. Chem. C 114 (2010), among others. It would be advantageous to provide a sensitized semiconductor for the photocatalytic reduction of CO2 under visible light which could potentially take advantage of impact ionization and produce higher conversion efficiencies than are possible with a dye-sensitized approach.
Semiconductor materials such as TiO2 have also been doped with transition metal ions and nonmetallic elements such as carbon, nitrogen, and sulfur in order to produce photocatalytic effects in the solar spectrum. Experimental and theoretical results indicate that these dopants generate localized energy levels in the TiO2 just above the valence band from which visible light excitation becomes feasible. See Konstantinova et al., “Carbon-Doped Titanium Dioxide: Visible Light Photocatalysis and EPR Investigation”, CHIMIA International Journal for Chemistry, Vol. 61, No. 12 (2007). Photocatalytic conversion of CO2 and H2O to hydrocarbons has been reported using nitrogen-doped TiO2 exposed to outdoor sunlight. See Varghese, et al., “High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon Fuels”, Nano. Lett. Vol. 9, No. 2 (2009). In these materials, the dopants are located in the lattice of the semiconductor material interstitially and/or substitutionally, in order to produce a doped semiconductor material which directly responds to visible light illumination. It would be advantageous to eliminate the interstitial and/or substitutional doping requirement and provide a sensitized semiconductor for the photocatalytic reduction of CO2 under visible light, in order to simplify synthesis and take advantage of characteristics such as impact ionization which may arise.
Doped semiconductor materials have also been utilized to produce composite semiconductor materials providing for oxidation and reduction reactions under visible light. See e.g., U.S. Pat. No. 7,169,733, issued to Wang et al, issued Jan. 30, 2007. Wang produces a composite material from two doped materials, both of which photoactivate under visible light activation. In this manner, the composite material is able to provide both a hole with high oxidation potential and an electron with high reducing potential. However, the composite material relies on doping as discussed above to provide visible light excitation of both materials involved, and does not rely on a sensitizer for the injection of electrons into a wide band gap semiconductor material.
It would be advantageous to provide a method whereby the photoreduction of CO2 under visible light illumination could be accomplished using an abundant and inexpensive wide band gap semiconductor material, such as TiO2 or ZnO. It would further be advantageous if visible light response was provided via electron injection from a sensitizing material responsive to visible light, in order to avoid doping requirements or other mechanisms designed to drive the wide band gap semiconductor itself into visible light photoresponse. It would be further advantageous to provide for the photoreduction of CO2 in a manner that avoid sensitization using dyes, so that sensitizer degradation from the UV fraction of the solar spectrum can be avoided, and so that impact ionization and higher conversion efficiencies can potentially be realized.
Accordingly, it is an object of this disclosure to provide a method of photocatalytically reducing CO2 under visible light excitation in the presence of hydrogen from a hydrogen source utilizing a sensitized photocatalyst comprised of a wide band gap semiconductor, a transition metal co-catalyst, and a semiconductor sensitizer.
It is a further object of this disclosure to photocatalytically reduce CO2 under visible light excitation in the presence of hydrogen from a hydrogen source in order to produce product molecules such as hydrocarbons, H2, and others.
It is a further object of this disclosure to control the composition of the product molecules based on the transition metal co-catalyst.
It is a further object of this disclosure to provide a method of photocatalytically reducing CO2 utilizing a sensitized photocatalyst comprised of particles of the wide band gap semiconductor, the transition metal co-catalyst, and the semiconductor sensitizer, in order to optimize charge injection and band alignments under visible light illumination.
It is a further object of this disclosure to provide a method of photocatalytically reducing CO2 under visible light illumination utilizing a sensitized photocatalyst in a CO2 and H2O environment.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.